1. Field of the Invention
This invention relates to a rolling-bearing unit, and more particularly to a rolling-bearing unit that is used in a location where there is high-speed and minute-rocking motion, for example, as in the bearing unit for the swing arm of a magnetic disk apparatus such as a Hard Disk Drive Apparatus (HDD), Flexible Disk Drive Apparatus (FDD), and also to a rolling-bearing unit with its radial rigidity controlled to a desired value for use in the magnetic disk apparatus.
2. Description of the Prior Art
As shown in Japanese Patent Publication No. TokuKai Hei 7-1 11053, the HDD for use in the memory device of computers etc. has a structure as shown in FIG. 1. When the HDD is used, the hard disk 101 is rotated at a high speed by an electric motor of the direct drive type. The swing arm 103 has a head 102 at its tip end, and is supported at its base end by a rolling bearing unit 104 as shown in FIG. 2 so that it swings with respect to the support shaft parallel to the rotation shaft of the hard disk 1.
The rolling bearing unit 104 as shown in FIG. 2 comprises an inner member or inner tube 5 in a cylindrical shape, an outer member or outer tube 6 in a cylindrical shape provided around the inner tube 5, and a pair of ball bearings 7 for supporting the inner tube 5 and the outer tube 6 such that they rotate relative to each other. The ball bearings 7 have an inner race 9 having an inner ring raceway 8 of a deep groove type or angular type formed on its outer peripheral surface, an outer race 11 having an outer ring raceway 10 of a deep groove type or angular type formed on its inner peripheral surface, and a plurality of rolling elements or balls 12 rotatively provided between the inner ring raceway 8 and the outer ring raceway 10. The balls 12 are rollably retained by a cage (not shown). In addition, although not illustrated, as required, the outer race 11 is formed with an attachment groove at the opposite ends of the inner peripheral surface generally along the circumference, and a shield plate is provided to have its outer peripheral edge attached to the attachment groove, so that the grease is prevented from leaking out of the space where the balls 12 are provided.
As shown in FIG. 3, a conventional bearing unit for the swing arm comprises a pair of ball bearings 100, 110 which are filled with grease, wherein a swing arm (not shown in the figure) is driven in a minute-rocking motion by a drive motor (not shown in the figure) having a rotor and a stator, and this swing arm is attached to a shaft 1 that is fastened on the inner periphery of the inner races 100A, 110A of the pair of ball bearings 100, 110, and a housing 2 is fastened around the outer periphery of the outer races 100B, 110B of the pair of ball bearings 100, 110.
This conventional rolling bearing unit is assembled such that it is possible to apply a pre-load to the pair of ball bearings 100, 110 in order to give them a specified radial rigidity, and such that it is possible to control runout of the shaft and achieve a specified positioning precision.
As shown in FIG. 3, in the assembly method for such a conventional rolling bearing unit, the lower end surface of the inner race 110A is supported by a jig 4, and a specified load from dead weight or spring force is applied to the upper end surface of the inner race 100A, then using a spacer 3, the position on the outer races 100B, 110B is fixed, and in this way a specified pre-load is applied by bringing the inner race 100A relatively close to the inner race 100A.
In this state, it is then possible to attach the outer peripheral portion of the shaft 1 with the inner peripheral surfaces 100a, 100a of the inner races 100A, 110A using adhesive or the like, as well as it is possible to attach the inner peripheral portion of the housing 2 with the outer peripheral surfaces 100b, 110b of the outer races 100B, 110B, also using adhesive or the like.
However, recently, there is an increasing demand for higher density magnetic disk apparatus.
Therefore, the width of the tracks on which signals are recorded on the disk has become narrower, and thus there is an even larger demand that the swing arm, in which the head for recording or reproducing the signals is mounted, be able to move at higher access speed to the target track and with improved positioning precision (faster and more precise control in positioning).
Therefore, together with an increasing demand for more rigidity of the ball bearing (rolling bearing) that supports the swing arm, there is a need to decrease the variation in rigidity between individual parts.
However, for a conventional rolling bearing unit for a swing arm, two bearings were selected from a bearing group, which were manufactured to fit within the tolerance range for a typical radial clearance, and with the method described above, the rolling bearing unit was assembled such that specified dead weight or spring force was applied in order to obtain the desired radial rigidity, however, there was a relatively large variation in the radial clearance and in the rate of curvature of the groove in the inner and outer races, so a relatively large variation in the rigidity between individual parts occurred.
It is thought to be possible to further increase the processing precision, so as to reduce the variation in the radial clearance and in the rate of curvature of the groove in the inner and outer races, and control the variation in the radial rigidity with a specified range, however, to at the present time, to actually do so would increase costs, that it is hard to adopt this way.
The ball bearings 7 in FIG. 2 and ball bearings in FIG. 3 have a similar structure. And, the following is a description referring to FIG. 2 only.
The ball bearings 7 as shown in FIG. 2 have a spacer 13 held between the outer races 11 which are fitted into the inner peripheral surface of the outer tube 6 and fixed with adhesion at two locations axially spaced apart from each other. In addition, the inner races 9 are fitted onto the outer peripheral surface of the inner tube 5 and fixed with adhesion at two locations axially spaced apart from each other in the state where a dead weight 14 is mounted on the vertically upper one of the inner races 9 or in the state where an axial load is applied to the inner races 9 in order that they come to each other. Accordingly, a pre-load is applied to the balls 12 at a contact angle in the opposite directions (face-to-face or back-to-back combination). The reason of applying the pre-load to the ball bearings 7 is to secure the rigidity of the ball bearings 7 and to improve the rotation precision.
In order to produce the rolling bearing unit with the pre-load controlled at an optimum value, the dead weight 14 or a spring, or a method as shown in FIG. 5, detailed later, is used to apply the predetermined axial load to the inner races 9. In the method of FIG. 5, the ball bearings 7 of the rolling bearing unit 4a are pressed in between the inner tube 5 and the outer tube 6 while the axial resonance frequency of the rolling bearing unit 4a is measured. And, at the point when the axial resonance frequency has reached a predetermined value, the ball bearings 7 are fixed between the inner tube 5 and the outer tube 6.
Specifically, the inner races 9 of the ball bearings 7 are pressed by a minute-motion feeding apparatus 15 so that they come close to each other while vibration is applied to the inner races 9 and the inner tube 5 by vibrators 17 provided between the lower surface of the minute-motion feeding apparatus 15 and the upper surface of a stage 16 provided with a load cell. Simultaneously, the axial resonance frequency of the rolling bearing unit 4a is measured by a sensor 18 which is provided in contact with or adjacent to a side surface of the outer tube 6. When the axial resonance frequency has reached a predetermined value, pressing the inner races 9 and therefore press-fitting the ball bearings 7 are stopped. It is possible to secure the axial rigidify at a desired value in the method of controlling the axial resonance frequency at a predetermined value because the axial rigidity corresponds to the axial resonance frequency. Also, it is possible to make the assembling process simple and easy because the ball bearings 7 are fitted to the inner tube 5 and the outer tube 6 in an interference fit relation. In the case of FIG. 5, in lieu of the spacer 13 in FIG. 2, a radially inward protrusion 19 is provided generally circumferentially on the inner peripheral surface of the outer race 6 at its middle portion.
In order to support the base end of the swing arm 103 (FIG. 1) with the rolling bearing unit 104, 4a in a rocking manner with respect to the support shaft, the inner tube 5 is fitted onto the support shaft, and a member such as E-block of the base end of the swing arm 103 is fitted onto the outer tube 6. And, a voice coil motor (VCM) is mounted on part of the member such as E-block to drive or rock the swing arm 103. In this state, the head 102 (FIG. 1) supported at the tip end of the swing arm 103 and close to the surface of the hard disk 101 (FIG. 1) moves complying with the surface of the hard disk 101 for signal reading and recording as the swing arm 103 rocks.
Recently, because of higher density achieved in memory devices such as HDDs, the width of the tracks on which signals are recorded for the hard disk 101 or flexible disk has become narrower. And the speed of reading and recording of the magnetic memory has become higher. Since the head 102 must trace at high speed and precisely the track having the very narrow width, the positioning precision and rocking speed corresponding to the rocking displacement of the swing arm 103 must be improved. To answer this need, the higher rigidity in the rolling bearing units 104, 4a, specifically the higher radial rigidity taking the rocking direction of the swing arm into consideration is required. However, it is difficult for the conventional rolling bearing units 104, 4a to satisfy the need.
There may be a variation in the pre-load applied to the pair of ball bearings 7 of the conventional rolling bearing units 104, 4a. Accordingly, in mass production of the rolling bearing units 104, 4a, the individual units may have larger or smaller radial rigidity. Specifically, when a predetermined axial load is applied to the ball bearings 7, for example with the dead weight 14 as shown in FIG. 4, the pre-load obtained is easily affected by the processing error of the inner ring raceway 8, outer ring raceway 10 and balls 12. More specifically, there are variations in the rate of curvature of the raceways 8, 10 and in the radial clearance based on the processing error of the inner ring raceway 8, outer ring raceway 10 and balls 12. And, if the predetermined axial load is always applied without taking into consideration these variations, the pre-load may be displaced out of the optimum range.
In addition, in the case of the rolling bearing unit 104 where the ball bearings 7 are fixed with adhesive while the predetermined axial load is applied, it is inconvenient to measure the pre-load applied to the ball bearings 7. Specifically, in order to determine whether the optimum pre-load is applied to the ball bearings 7, the axial resonance frequency of the rolling bearing unit 104 is measured after the assembly of the rolling bearing unit 104 is completed, by applying impact it in the free state (impact vibrating). Since the measurement step and the assembly step are separated from each other, the production process of the rolling bearing unit 104 is complicated, which increases the production cost.
On the other hand, in the case of assembling the rolling bearing unit 4a while the axial resonance frequency is applied as shown in FIG. 5, since the measurement step of the axial resonance frequency is made simultaneously with the assembly step, the production cost is lower. However, the variation in preload based on the processing error as mentioned above could not be avoided, and the individual units may have larger or smaller radial rigidity, that is there may be a variation in radial rigidity.
In order to clarify this point, the inventor examined the relation between the pre-load, axial rigidity and radial rigidity with respect to the rolling bearing unit having a pair of ball bearings with larger radial clearance and the rolling bearing unit having a pair of ball bearings with smaller radial clearance.
The result is explained referring to FIG. 6.
In FIG. 6, the solid line {circle around (1)} indicates the axial rigidity of a rolling bearing unit having a pair of ball bearings with smaller radial clearance, and the dotted line {circle around (2)} indicates the radial rigidity of the rolling bearing unit having the pair of ball bearings with the smaller radial clearance.
In addition, the single-dot chain line {circle around (3)} indicates the axial rigidity of a rolling bearing unit having a pair of ball bearings with larger radial clearance, and the two-dot chain line {circle around (4)} indicates the radial rigidity of the rolling bearing unit having the pair of ball bearings with the larger radial clearance. As clear from FIG. 6, the axial rigidity or the axial resonance frequency corresponding to the axial rigidity is at a fixed value, the values in radial rigidity may be largely different depending on the size of the radial clearance.
Specifically, when the pair of ball bearings with the smaller radial clearance and the pair of ball bearings with larger radial clearance are installed respectively with the same axial resonance frequency corresponding to the axial rigidity J, for example, the pre-load applied to the ball bearings with smaller radial clearance is at the value xe2x80x9cxcex1xe2x80x9d corresponding to the solid line {circle around (1)}, and the pre-load applied to the ball bearings with larger radial clearance is at the value xe2x80x9cxcex2xe2x80x9d corresponding to the single-dot chain line {circle around (3)}. As to the radial rigidity of the ball bearings corresponding to the pre-loads xe2x80x9cxcex1xe2x80x9d and xe2x80x9cxcex2xe2x80x9d, the radial rigidity of the ball bearings with larger radial clearance is at the value xe2x80x9cxcex3xe2x80x9d corresponding to the two-dot chain line {circle around (4)}, and the radial rigidity of the ball bearings with smaller radial clearance is at the valuexe2x80x9cxcex4xe2x80x9d corresponding to the dotted line {circle around (2)}.
Accordingly, although the axial rigidity (axial resonance frequency) is constant, the larger in radial clearance the smaller in radial rigidity, and the smaller in radial clearance the larger in radial rigidity, resulting in that there may be a variation in radial rigidity by the difference xe2x80x9cXxe2x80x9d between the xe2x80x9cxcex3xe2x80x9d and xe2x80x9cxcex4xe2x80x9d Accordingly, during mass production of the rolling bearing unit with the axial rigidity (axial resonance frequency) fixed at the value xe2x80x9cJxe2x80x9d, there may be a variation between the values xe2x80x9cxcex3xe2x80x9d and xe2x80x9cxcex4xe2x80x9d in the radial rigidity of the rolling bearing unit depending on the size of radial clearance in the ball bearings caused by processing error etc.
Although there may be a way to make the radial clearance as small as possible by improving the process precision of the inner ring raceway 8, outer ring raceway 10 and balls 12 so as to decrease the variation as mentioned above, it is not desirable due to increase in process time and cost etc.
Taking the aforementioned problems into consideration, it is an object of this invention to provide a rolling-bearing unit and an assembly method for that rolling-bearing unit that is simple as well as low cost, and which further makes the radial rigidity more uniform and more rigid by reducing the variation in the radial rigidity between individual parts.
Another object of this invention is to provide a rolling bearing unit stably at a lower cost, with its radial rigidity controlled at a desired value, such that the variation in radial rigidity is very small even in mass production.